The purpose of this study is to investigate the cause of PM reduction by diesel fuel (Solvent:C₁₂, C₁₃, C₁₄) mixed with bio-diesel fuel (Methyl Decanoate:C₉H₁₉COOCH₃) with focusing on the thermal decomposition in diesel combustion atmosphere. To make a heavy duty diesel combustion atmosphere, a flow reactor and a co-flow diffusion burner were used. The thermally decomposed fuel produced in the flow reactor was introduced into the co-flow diffusion burner directly. To study the relation between thermal decomposition and PM reduction, the chemical substances produced in the reactor and PM produced by the combustion of the burner were quantitatively and qualitatively analyzed by some analytical devices. The emission measurement with diffusion burner shows that the bio-diesel blended fuel exhausts PM in concentration 20-30% less than diesel fuel. The analysis indicates that, due to oxygen-containing fuel, the bio-diesel fuel can oxidize thermally decomposed components of C₂H₂ and PAHs that may become pre-cursers of PM.